1. Field of the Invention
The present invention generally relates to audio time scale modification algorithms.
2. Background
In the area of digital video technology, it would be beneficial to be able to speed up or slow down the playback of an encoded audio signal without substantially changing the pitch or timbre of the audio signal. One particular application of such time scale modification (TSM) of audio signals might include the ability to perform high-quality playback of stored video programs from a personal video recorder (PVR) at some speed that is faster than the normal playback rate. For example, it may be desired to play back a stored video program at a 20% faster speed than the normal playback rate. In this case, the audio signal needs to be played back at 1.2× speed while still maintaining high signal quality. However, the TSM algorithm may need to be of sufficiently low complexity such that it can be implemented in a system having limited processing resources.
One of the most popular types of prior-art audio TSM algorithms is called Synchronized Overlap-Add, or SOLA. See S. Roucos and A. M. Wilgus, “High Quality Time-Scale Modification for Speech”, Proceedings of 1985 IEEE International Conference on Acoustic, Speech, and Signal Processing, pp. 493-496 (March 1985), which is incorporated by reference in its entirety herein. However, if this original SOLA algorithm is implemented as is for even just a single 44.1 kHz mono audio channel, the computational complexity can easily reach 100 to 200 mega-instructions per second (MIPS) on a ZSP400 digital signal processing (DSP) core (a product of LSI Logic Corporation of Milpitas, Calif.). Thus, this approach will not work for a similar DSP core that has a processing speed on the order of approximately 100 MHz. Many variations of SOLA have been proposed in the literature and some are of a reduced complexity. However, most of them are still too complex for an application scenario in which a DSP core having a processing speed of approximately 100 MHz has to perform both audio decoding and audio TSM.
Accordingly, what is desired is a high-quality audio TSM algorithm that provides the benefits of the original SOLA algorithm but that is far less complex, such that it may be implemented in a system having limited processing resources.